I know you're very keen on the idea of moving away from Windows. However, it seems like your family may at least be used to that version or installation of it. As a result, you may be causing more trouble than you want by giving them a new interface that's more foreign than what they have now.
Therefore, I might suggest a $45 option to "harden" a Windows installation: Faronics: Deep Freeze. (http://www.faronics.com/products/deep-freeze/standard/)
This program will let you get their existing PC setup in that perfect state you want it in, then 'freeze' it in place. If they get a virus, spyware, malware, etc.. The solution is simple: Reboot. Upon rebooting, the PC -always- reverts to the state you froze it in. Their document storage space they can read/write and things stay there, but if a program is installed, *poof* it's gone.
While it might not be a perfect solution for you, you'll be able to make a yearly trip to do updates, or use 'normal' PC remote control methods to run updates and re-Freeze the system.
At any rate, something to consider.
Good luck, either way!